Wrist pin and method of reducing wear between members thereof, connecting rod, piston and methods of constructing same

ABSTRACT

A wrist pin and method of reducing wear between members thereof, a piston, a connecting rod, and methods of construction thereof are provided. The wrist pin has a first portion extending between opposite ends configured for receipt in piston pin bores and a second portion extending between opposite ends configured for receipt in the pin bores. The second portion is configured for relative movement with the first portion. The piston includes a piston body having axially aligned pin bores, wherein at least one of the pin bores has a feature to prevent relative rotation of a wrist pin member. The connecting rod includes a connecting rod body having small end bore. The small end bore has a contour to allow relative rotation of a first wrist pin member therein and a feature to prevent relative rotation of a second wrist pin member therein.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional application of U.S. Divisionalapplication Ser. No. 13/434,700, filed Mar. 29, 2012 and of U.S.application Ser. No. 12/488,853, filed Jun. 22, 2009, which claimspriority to U.S. Provisional Application Ser. No. 61/075,075, filed Jun.24, 2008, which are incorporated herein by reference in their entirety.

BACKGROUND OF THE INVENTION

1. Technical Field

This invention relates generally to piston assemblies, and moreparticularly to wrist pins, pistons, connecting rods and pistonassemblies therewith and to their respective methods of construction andassembly.

2. Related Art

Internal combustion engines are known to have piston assembliesincluding a piston, a connecting rod and a wrist pin. The wrist pinattaches the piston to the connecting rod for translation of the pistonwithin a cylinder bore in response to rotation of a crankshaft andcombustion within the cylinder bore. Typically, the piston has a pair ofpin bosses with axially aligned cylindrical pin bores configured forreceipt of cylindrical ends of the wrist pin. The connecting rod has oneend configured for attachment to a crankshaft and a small end with acylindrical small end bore configured for receipt of the cylindricalwrist pin therethrough. Accordingly, the wrist pin couples theconnecting rod to the piston.

In an effort to reduce friction between the wrist pin and the pin boresand the wrist pin and the small end bore of the connecting rod,typically a suitable bearing material is incorporated in the pin boresand the small end bore. Although the bearing material can assist inreducing friction, inevitably wear results to the bearing surfaces ofthe wrist pin and the respective bores. The wear generally results dueto a lubrication film breakdown between the bearing surfaces. Thelubrication film breakdown results generally from the excessivepressures generated between the bearing surfaces, the relative rotatingand/or oscillating movement between the bearing surfaces, aninsufficient supply of lubrication reaching the bearing surfaces andfrom exposure of the bearing surfaces and the lubrication to high heat.

In addition to ultimate failure of the bearing surfaces, anotherundesirable phenomenon typically results from the breakdown oflubrication. Noise typically is generated between the bearing surfacesof the piston pin bores, the wrist pin and the connecting rod when thelubrication film is diminished. The lubrication film breakdown resultsin excessive clearance between the components, and thus, insufficientdamping occurs between mating bearing surfaces.

In an effort to minimize the negative consequences discussed above,considerable care is generally taken during manufacture of the bearingsurfaces of the piston pin bores, the connecting rod small end bore andthe wrist pin. In order to minimize breakdown of the lubrication film,more clearance needs to be incorporated between the respective bearingsurfaces. However, in order to reduce the onset of noise, the clearancebetween the respective bearing surfaces needs to be minimized. As such,in order to attain the enhanced lubrication and reduced noise generationdesired, precise diameters and surfaces finishes of the bearing surfacesover a relatively narrow tolerance range need to be attained, such as intightly controlled machining operations, e.g., grinding operations. Assuch, the costs associated with the manufacture of these parts isrelatively high in comparison to parts requiring less precision.Although the manufacturing processes have improved, thereby allowing formore precise fabrication over narrow tolerance ranges, the everincreasing demands of modern engines, including higher cylinderpressures and engine speeds, and the ever increasing environmentalregulations for reduced engine emissions and increased fuel economy,make it difficult or impossible to attain clearances and sufficientsurface finishes between the respective bearing surfaces to combat theoperating environment of the modern engine while in use, such as extremepressures and temperatures, for example.

SUMMARY OF THE INVENTION

According to one aspect of the invention, a wrist pin configured forreceipt in axially aligned pin bores of a piston body to couple aconnecting rod to the piston body is provided. The wrist pin has a firstportion extending between opposite ends configured for receipt in thepin bores and a second portion extending between opposite endsconfigured for receipt in the pin bores. The second portion isconfigured for relative movement with the first portion.

In accordance with yet another aspect of the invention, a piston isprovided. The piston includes a piston body having a pair laterallyspaced pin bosses. The pin bosses have axially aligned pin bores,wherein at least one of the pin bores has a feature to prevent relativerotation of a wrist pin member.

In accordance with yet another aspect of the invention, a connecting rodfor operable attachment to a piston is provided. The connecting rodincludes a connecting rod body having small end bore. The small end borehas a contour to allow relative rotation of a first wrist pin membertherein and a feature to prevent relative rotation of a second wrist pinmember therein.

In accordance with yet another aspect of the invention, a method ofconstructing a piston is provided. The method includes forming a pistonbody with an upper crown and a pair of laterally spaced pin bossesdepending from the upper crown. Further, forming a wrist pin bore ineach of the pin bosses. Further yet, providing a feature within at leastone of the wrist pin bores to prevent relative rotation of a wrist pinmember within the wrist pin bores.

In accordance with yet another aspect of the invention, a method ofconstructing a wrist pin for receipt in axially aligned pin bores of apiston body is provided. The method includes forming a first portionextending between opposite ends configured for receipt in the pin boresand forming a second portion extending between opposite ends configuredfor receipt in the pin bores wherein the second portion is formed from aseparate piece of material from the first portion. Further, configuringthe first portion for engagement with the second portion and forrelative movement with the second portion.

In accordance with yet another aspect of the invention, a method ofconstructing a connecting rod for operable attachment to a piston isprovided. The method includes forming a connecting rod body and forminga small end bore in the body. Then, forming a portion of the small endbore with a contour to allow relative rotation of a first wrist pinmember, and further, providing a feature in another portion of the smallend bore to prevent relative rotation of a second wrist pin member.

In accordance with yet another aspect of the invention, a method ofreducing wear between first and second wrist pin members of a wrist pinis provided. The method includes collecting sufficient oil regardless ofan existing clearance value between the wrist pin and a piston pin boreand regardless of an existing clearance value between the wrist pin anda small end bore of a connecting rod. Further, increasing the contactarea between the first and second wrist pin members to reduce contactpressure and enable improved lubrication and damping characteristicsbetween the first and second wrist pin members. Further yet, providing alubrication interface between the first and second wrist pin members tomaintain supplied oil with limited contact pressure between the firstand second wrist pin members and, providing a reduced sliding speedbetween the first and second wrist pin members.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects, features and advantages of the invention willbecome more readily appreciated when considered in connection with thefollowing detailed description of presently preferred embodiments andbest mode, appended claims and accompanying drawings, in which:

FIG. 1 is a partial cross-sectional perspective view of a pistonassembly constructed according to one aspect of the invention;

FIG. 2A is a cross-sectional view taken along a central pin axis of thepiston assembly of FIG. 1;

FIG. 2B is an alternate embodiment of the encircled area 2B of FIG. 2A;

FIG. 3 a cross-sectional view of a wrist pin constructed according toone presently preferred aspect of the assembly;

FIG. 4 is an end view of the wrist pin of FIG. 2A with a lock memberremoved;

FIG. 5 is a perspective end view of one portion of the wrist pin of FIG.4;

FIG. 6 is a perspective end view of the wrist pin of FIG. 4 with thelock member capturing one portion of the wrist pin axially relative toanother portion of the wrist pin;

FIG. 7 is a cross-sectional side view of the piston assembly of FIG. 1;and

FIG. 8 is a view similar to FIG. 7 with the wrist pin and connecting rodof the assembly removed.

DETAILED DESCRIPTION OF PRESENTLY PREFERRED EMBODIMENTS

Referring in more detail to the drawings, FIG. 1 illustrates a pistonassembly 10 constructed according to one aspect of the invention. Thepiston assembly 10 has a piston body 12, preferably either cast orforged, although any preferred method of construction could be used,having an upper crown portion 14 formed with ring grooves 16 toaccommodate one or more rings (not shown). A pair of pin bosses 18 (FIG.2A) depend from the upper crown portion 14 and terminate in pin bores 20axially aligned with one another along an axis 22. The pin bores 20 areconfigured for receipt of a wrist pin 24 therein. The pin bosses 18 haveinner and outer faces 26, 27, wherein the inner faces 26 are laterallyspaced from one another a distance sufficient to receive a connectingrod 28, and more particularly, a small end 30 of the connecting rod 28therebetween. The small end 30 of the connecting rod 28 has a wrist pinor small end bore 32 extending along the axis 22 and is configured forreceipt of the wrist pin 24 therethrough. Accordingly, the wrist pin 24couples the connecting rod 28 to the piston body 12. The wrist pin 24 isconstructed as a two-piece member, and thus, has first and second wristpin members, also referred to as portions, constructed from separatepieces of material, referred to hereafter simply as first and secondportions 38, 40, respectively. The first and second portions 38, 40 areelongate members that are arranged to extend substantially parallel tothe axes 22, 34 through the small end bore 32 and are configured to moverelative to one another during reciprocation of the piston body 12 inthe cylinder bore. The relative movement between the first and secondportions 38, 40 is a reduced sliding motion, and thus, any relativesliding movement between the first and second portions 38, 40 isminimized. Further, as discussed in more detail below, the relativemovement between the pin bores 20 and the first portion 38 of the wristpin 24 is prevented, and the relative movement between the small endbore 32 and the second portion 40 of the wrist pin 24 is prevented,thereby minimizing or negating the potential for wear therebetween. Inaddition, the ability to maintain a lubrication film between thecomponents while moving relative to one another, i.e. first and secondportions 38, 40, is maximized, thereby further reducing the potentialfor wear therebetween, while also minimizing the potential forgenerating noise and heat.

As shown in FIG. 3, both the first portion 38 and second portion 40 ofthe wrist pin 24 are non-circular in lateral cross-section, and thus,non-cylindrical along their lengths. Accordingly, the wrist pin 24 doesnot have a contiguous, cylindrical outer surface. Any suitablemanufacturing process could be used to construct the initial shaped ofthe first and second portions 38, 40, such as a roll forming extrusionprocess, for example. The first portion 38 has a convex upper surface 42having a first radius r1 extending along a predetermined arc betweenopposite ends 43, 45, wherein the arc is represented as being less than180 degrees, and between about 60-145 degrees, for example. The uppersurface 42 transitions radially inwardly to a necked down portion 44 toprovide a pair of opposite longitudinally extending recessed flutes orpockets 46, such that the first portion 38 is generally mushroom-shapedin lateral cross-section. The necked down portion 44 extends to an endproviding a convex fulcrum 48 having a second radius r2 extending alongthe length of the first portion 38. The second radius r2 can be providedas a constant or varying radius, as desired.

The upper surface 42 of the first portion 38 can remain as an “initiallyformed” surface, and thus, does not require secondary machiningprocesses. However, if secondary operations are desired, such asmachining, the upper surface 42 can remain relatively rough, having asurface finish of no more than about Ra 4 μm, for example.

The convex fulcrum 48 of the first portion 38, in contrast to the uppersurface 42, is preferably formed as a smooth surface, such as in amachining operation, e.g. grinding. The resulting surface finish of theconvex fulcrum is preferably no greater than about Ra 2 μm, for example.

The region of the necked down portion 44 other than the convex fulcrum48 can remain as initially formed. Accordingly, the pockets 46 canremain as initially formed, thereby not requiring any secondarymachining.

The second portion 40 has a convex lower surface 50 having a thirdradius r3 extending along a predetermined arc between opposite ends 51,53, wherein the arc is represented as being less than 180 degrees, andin one example, as being between about 60-145 degrees. The lower surface50 transitions radially inwardly to a necked down portion 52 to providea pair of opposite longitudinally extending recessed flutes or pockets54. The necked down portion 52 transitions to a flared, generallyY-shaped portion 56 providing a concave surface 58 extending along thelength of the second portion 40. The concave surface 58 can be providedas a constant or varying radius, as desired, and is preferably shaped toprovide a reduced sliding motion relative to the convex fulcrum 48 ofthe first portion 38. As shown in FIGS. 3 and 4, the concave surface 58can be provided having a constant radius portion r4, wherein the radiusr4 is equal to or substantially equal to the radius r2, therebyestablishing a single elongate contact patch 62 between the first andsecond portions 38, 40 over an arc of up to about 40 degrees, which inturn, minimizes the contact pressure therebetween.

The concave surface 58, aside from establishing a predetermined shapeand size contact patch with the convex fulcrum 48, also provideslubrication pockets 64 laterally outwardly from the contact patch 62.Accordingly, the concave surface 58 is formed having an included angle Athat is greater than an included angle B formed by the convex fulcrum48. As such, a substantial space is provided between the mating firstand second portion 38, 40 in which lubrication can collect and remain inuse, thereby further inhibiting or preventing scuffing, while alsoacting to dampen noise, as discussed further below.

The lower surface 50 of the second portion 40 can remain as an“initially formed” surface, and thus, does not require secondarymachining processes. However, if secondary operations are desired, suchas machining, the lower surface 50 can remain relatively rough, having asurface finish no greater than about Ra 4 μm, for example.

The concave surface 58 of the second portion 40, in contrast to thelower surface 50, is preferably formed as a smooth surface, such as in amachining operation, e.g. grinding. The resulting surface finish of theconcave surface 58 is preferably no greater than about Ra 2 μm, forexample.

The region of the necked down portion 52 can remain as initially formed.Accordingly, the pockets 54 can remain as initially formed, thereby notrequiring any secondary machining.

As shown in FIG. 2A, by way of example and without limitation, the firstand second portions 38, 40 can be formed having different lengths,wherein the first portion 38 is shown as being slightly shorter than thesecond portion 40. With the second portion 40 ends 51, 53 extendingbeyond the ends 43, 45 of the first portion 38, lock devices, such asspring or lock washers 66, for example, can be incorporated on the ends51, 53 of the longer second portion 40 to retain the second portion 40against axial movement relative to the first portion 38. As shown inFIGS. 4-6, by way of example and without limitation, the lock washers 66can be received in slots 68 recessed into the opposite ends 51, 53 ofthe second portion 40. The slots 68 are shown here as having a dovetailtype form, such that the washers 66, upon being snapped into the slots68, are maintained therein against axial disassembly. To preventrelative radial movement of the washers 66 within the slots 68, thewashers 66 can be formed with laterally extending tabs 69 that arereceived and radially captured in the pockets 54. The washers 66 aresuitably sized to extend radially outwardly from the concave surface 58to abut or present barriers to the opposite ends 43, 45 of the firstportion 38.

As shown in FIG. 2A, the pin bores 20 can be formed having a steppedregion 70 extending partially about their circumference. The steppedregions 70 are formed adjacent the outer faces 27, such that a lip orflange is formed extending radially inwardly from the pin bores 20. Theflange formed by the stepped regions 70 facilitates maintaining thefirst and second portions 38, 40 of the wrist pin 24 in a fixed axiallocation within the pin bores 20. By way of example and withoutlimitation, the stepped regions 70 are shown here as extending radiallyinwardly from an upper portion 72 of the pin bores 20 to capture thefirst portion 38 of the wrist pin axially, thereby preventing the firstportion 38 from moving axially, while a lower portion 74 of the pinbores 20 are formed as straight through bores without a stepped surface.The upper or first portion 38 of the wrist pin 24 can be inserted intothe pin bores 20 and positioned upwardly between the laterally spacedstepped regions 70, and then the lower or second portion 40 of the wristpin 24 can be inserted into the pin bores 20 to radially maintain thefirst portion 38 within the pin bores 20 between the stepped regions 70.The second portion 40 can then be maintained axially within the pinbores 20 by assembling the lock washer 66 in the recessed slots 60 ofthe second portion 40, for example.

It should be recognized that the pin bores 20 could be formed withoutthe stepped regions 70 and as circumferential through bores, and thatstandard snap rings could be disposed within snap ring grooves (notshown) within the pin bores 20 to fix the wrist pin 36 against relativeaxial movement out from the pin bores 20. In addition, as shown in FIG.2B, by way of example and without limitation, rather than the steppedregions 70 being provided, a contoured surface 76 could be formed withinthe pin bores 20. The contoured surfaces 76 are shown here as convergingslightly toward the outer faces 27 such that the openings formed at theouter faces of the pin bores is at least slightly reduced from the sizeof the pin bores axially inwardly from the outer faces 27. The contouredsurfaces 76 are represented here as being curvilinear to maintain thefirst portion 38 of the wrist pin against axial movement out of the pinbores 20. As with the stepped regions 70, the contoured surfaces 76extend only partially about the circumference of the respective pinbores 20 such that the lower portion 74 is formed as an axially straightthrough bore.

As shown in FIG. 8, the pin bores 20, in one example according to afirst contemplated construction of the piston assembly 10, arenon-cylindrical with one portion, represented here as an upper portion78, by way of example and without limitation, having a first concaveradius of curvature R1, and another portion, represented here as a lowerportion 80 diametrically opposite the upper portion 78, by way ofexample and without limitation, having a second concave radius ofcurvature R2, wherein R1 is generally equal to or less than R2. Theupper and lower portions 78, 80 each extend less than 180 degrees aboutthe complete respective bore 20 and are generally symmetrical onopposite sides of the axis 22. Each portion 78, 80 terminates at aradially inwardly extending protrusion 82. The protrusions 82 interruptthe radius contour of the upper and lower portions 78, 80 and extendradially inwardly in mirrored relation to one another. As such, thebores 20 are generally hour-glass shaped or FIG. 8 shaped.

As shown in FIG. 7, the small end bore 32 of the connecting rod 28 isnon-cylindrical, with one portion, represented here as an upper portion84 forming an end 85 of the connecting rod 28, having a first concaveradius of curvature Cr1, and another portion, represented here as alower portion 86 diametrically opposite the upper portion 84 having asecond concave radius of curvature Cr2. The upper portion curvature Cr1,in the embodiment shown, by way of example and without limitation, isgenerally equal to or less than the lower portion curvature Cr2. Theupper and lower portions 84, 86 each extend less than 180 degrees aboutthe small end bore 32 and transition to a radially inwardly extendingsurface, wherein the radially inwardly extending surface is referencehereafter as an intermediate portion 88. The intermediate portion 88 isformed in a generally symmetrical fashion on opposite sides of the smallend bore 32, and thus, the small end bore 32 has a generally mirroredshape across an axis of its length, and is generally hour-glass shapedor FIG. 8 shaped.

The connecting rod 28 is preferably cast or forged from steel, althoughother materials having a suitable strength for the intended applicationare contemplated, such as aluminum, for example, and other methods ofconstruction, e.g. machining, are contemplated. The connecting rod smallend bore 32 can remain as manufactured, and thus, can have a relativelyrough surface finish, preferably no greater than about Ra 4 μm, forexample. Accordingly, no secondary operations are necessary in themanufacture of the small end bore 32. In addition, the small end bore 32does not require a supplemental bearing surface or material, and thus,is preferably provided as a bushingless bore, thereby being economicalin manufacture.

In use, the piston assembly 10, such as in a four-stroke engine, thoughthe assembly is equally well suited for a two-stroke engine,reciprocates through a full-stroke (intake, compression, combustion andexhaust) with no relative movement occurring between the first portion38 of the wrist pin 36 and the pin bores 20, nor between the secondportion 40 of the wrist pin 24 and the small end bore 32. The firstportion 38 of the wrist pin 36, being received and substantially fixedin the upper portions 78 of the pin bores 20, is constrained againstrelative rotational movement within the pin bores 20 by the protrusions82 of the pin bores 20. The upper portions 78 of the pin bores 20 spanthe same or substantially the same arc as the upper surface 42 of thefirst portion 38, with the protrusions 82 of the pin bores 20 acting asstops or barriers against relative rotational movement of the wrist pinfirst portion 38. In addition to the barriers provided by theprotrusions 82, the relatively rough surfaces of the upper portion 78 ofthe pin bores 20 and the upper surface 42 of the first portion 38 whileengaging one another and act to resist relative rotation throughrelatively high friction therebetween.

In addition, the second portion 40 of the wrist pin 36, being receivedin the lower portion 86 of the connecting rod small end bore 32, issubstantially fixed and constrained against rotational movement relativeto the small end bore 32 by the intermediate portion 88 of the small endbore 32. The lower portion 86 of the small end bore 32 spans the same orsubstantially the same arc as the lower surface 50 of the second portion40, with the intermediate portion 88 of the small end bore 32 acting asa stop or barrier against relative rotational movement of the wrist pinsecond portion 40. In addition to the barrier provided by theintermediate portion 88, the relatively rough surfaces of the lowerportion 80 of the pin bores 20 and the lower surface 50 of the secondportion 40 while engaging one another act to resist relative rotationthrough relatively high friction therebetween.

Accordingly, the relative movement between the connecting rod 28 and thepiston body 12 occurs between the first portion 38 of the wrist pin 24and the second portion 40 of the wrist pin 24, wherein the relativemovement is a reduced sliding motion of the concave surface 58 againstthe convex fulcrum 48. The reduced sliding motion takes place over thefull length of contact between the concave surface 58 and the convexfulcrum 48, which is represented here as extending across the entirelength between the pin bores 20. Accordingly, the area of contactbetween the relatively movable first and second portions 38, 40 providesrelatively low contact pressure between the convex fulcrum 48 and theconcave surface 58, and has been found to be, in one example withoutlimitation, wherein AISI 4140 steel was used to construct the pins,about 320 MPa. As such, the potential for scuffing and wear between therelatively movable first and second portions 38, 40 of the two componentwrist pin 24 is minimized or substantially eliminated. The potential forwear and noise generation is further reduced by the presence oflubrication within the elongate lubrication pockets 64 extending betweenthe pin bores 20 on both sides of the convex fulcrum 48. Given therelatively low contact pressures between the first and second portions38, 40, a sufficient lubrication film thickness is provided andmaintained between the first and second portion 38, 40 to further reducethe potential for wear. In addition to minimizing wear, the lubricationfilm between the convex fulcrum 48 and the concave surface 58 acts todampen noise.

Throughout the complete stroke of the piston assembly 10, differentcompression and tension loads are transmitted throughout the pistonassembly 10. For example, during an initial intake stroke, theconnecting rod 28 is placed in tension. While in tension, the connectingrod 28 pulls on the upper surface 42 of the wrist pin first portion 38which is constrained against relative rotation with the pin bores 20.Accordingly, any relative rotation between the connecting rod 28 and thewrist pin 24 occurs between the upper surface 42 of the first portion 38and the upper portion 84 of the small end bore 32. However, the tensileforces generated are relatively low, and thus, the potential for wearbetween the relatively rotating surfaces 42, 84 is minimal. The sameholds true for any relative rotational movement between the lowerportion 80 of the pin bores 20 and the lower surface 50 of the wrist pinsecond portion 40.

The connecting rod 28 is also placed in compression, which is thecondition under which the most severe loading, and thus, the greatestpotential for wear exists, such as during the compression, combustionand exhaust strokes. While under compression, the connecting rod 28 andpiston body 12 are being pushed toward one another, with the wrist pin24 transferring the loading therebetween. The wrist pin first portion 38encounters compression forces in the pin bores 20 and the wrist pinsecond portion 40 encounters compression forces in the connecting rodsmall end bore 32. With both the first and second wrist pin portions 38,40 being placed under compressive loads acting toward one another, andwith the first portion 38 being fixed against rotation in the pin bores20 and the second portion 40 being fixed against rotation in the smallend bore 32, the first and second wrist pin portions 38, 40 are causedto move in a reduced sliding motion between the convex fulcrum 48 andthe concave surface 58, respectively. With the movement being a minimalsliding type motion, and with the reduced pressures resulting across theentire axial length of the sliding surfaces, as discussed above, andfurther, with the enhanced levels of lubrication and lubrication filmbeing maintained across the entire axial length of the sliding surfaces,the potential for wear and noise is greatly reduced.

As such, in accordance with another aspect of the invention, a method offorming an enhanced lubrication interface between first and second wristpin members 38, 40 of a wrist pin 24 is provided. The method includescollecting sufficient oil regardless of the existing clearance valuebetween the wrist pin 24 and a piston pin bore 20, and regardless of anexisting clearance value between the wrist pin 24 and a small end bore32 of a connecting rod 28. Further, the method includes increasing thecontact area between the first and second wrist pin members 38, 40 toreduce contact pressure and enable improved lubrication and dampingcharacteristics between the first and second wrist pin members 38, 40;and then, providing a lubrication interface between the first and secondwrist pin members 38, 40 to maintain supplied oil with limited contactpressure between the first and second wrist pin members 38, 40, andfurther, providing a reduced sliding speed between compatible first andsecond wrist pin materials 38, 40 to prevent their surface damage.

Obviously, many modifications and variations of the present inventionare possible in light of the above teachings. It is, therefore, to beunderstood that within the scope of the appended claims, the inventionmay be practiced otherwise than as specifically described.

What is claimed is:
 1. A piston, comprising: a piston body having a pairof laterally spaced pin bosses with pin bores aligned with one anotheralong an axis and configured for receipt of a wrist pin having separatefirst and second portions, said first and second portions each extendinglengthwise between said pin bores, with at least one of said pin boresso as to be received by both pin bores having a feature to preventrelative rotation of the first wrist pin portion.
 2. The piston of claim1 wherein said pin bores are configured to allow relative rotation ofthe second wrist pin portion.
 3. The piston of claim 1 wherein saidfeature extends radially inwardly toward said axis.
 4. The piston ofclaim 3 wherein said feature is provided in each pin bore by a pair ofprotrusions extending radially inwardly generally toward said axis. 5.The piston of claim 1 further including providing a feature in said pinbores to prevent relative axial movement of the first wrist pin portionin said pin bores.
 6. A method of constructing a piston, comprising:forming a piston body having an upper crown and a pair of laterallyspaced pin bosses depending from said upper crown; forming a wrist pinbore in each of said pin bosses for receipt of a wrist pin having firstand second portions extending lengthwise between the pin bores so as tobe received by both pin bores; and providing a feature within at leastone of said wrist pin bores to prevent relative rotation of the firstwrist pin portion therein.
 7. The method of claim 6 further includingforming a portion of each wrist pin bore for relative rotation with thesecond wrist pin portion.
 8. The method of claim 6 further includingproviding said feature by forming a protrusion extending radially intosaid at least one wrist pin bore.
 9. The method of claim 8 furtherincluding forming said protrusion as a monolithic piece of material withsaid pin boss.
 10. The method of claim 6 further including forming eachof said wrist pin bores having one portion with a first radius ofcurvature and a second portion with a second radius of curvature, saidfirst radius of curvature being different than said second radius ofcurvature.